In apparatus for aerating sewage, for example, it is common to provide a submerged grid of air supply pipes supporting a plurality of porous ceramic diffusers which discharge the air in the form of fine bubbles to rise through the liquid sewage body for aerating the same. The pipes and diffuser supported thereon are preferably arranged so that the degree of aeration is greater near the sewage inlet and decreases toward the outlet where less aeration is required. It is essential that the effluent surfaces of the various diffusers be arranged at the same level, within a close tolerance, so that the head of liquid thereon is substantially uniform.
Heretofore, the diffuser surface density per unit of tank area was planned in advance of installation of the equipment by spacing diffusers at different intervals along the air supply pipe. After installation in some instances it was found from actual operating experience that the diffuser surface density was not proper. Usually too much aeration was found to have been supplied, resulting in excessive power cost. In order to reduce the power cost, curtailing the aeration by throttling the air supply had practical limitations. The situation was typically corrected by draining the tank and either replacing the pipe with diffusers at a different spacing, or modifying the pipe by plugging some diffuser service holes and providing new holes, with attendant diffuser mounting difficulties. Sometimes the diffusers were replaced with ones of different size but the construction of diffuser units and its mounting mode was such that special cleaning and special tools were required.
From time to time in the operation of a diffused aeration system for wastwater using ceramic fine bubble equipment, a power failure occurs and the supply of gas such as air is cut off. Back pressure in the system drains liquid and suspended solids, such as activated sludge, through the diffuser and into the air pipework and in the air plenum beneath the diffuser. On restoration of air supply, this regressed liquid and susended solids mixture is blown back through the pipework. However, because prior art diffusers were so designed, either when mounted on the crown of a pipe or with a center bolt hold-down construction, recessed or dead pocket cavities existed within the air plenum where residual liquids or solids accumulated. The residual liquid evaporated leaving the suspended and dissolved solids in those cavities. On restoration of the air supply, these accumulated solids could be blown against the underside or influent side of the diffuser elements causing air-side fouling of the diffusers with an attendant loss in efficiency.